1. Field of the Invention
The present invention relates to a fuel vapor emission control system for an engine.
2. Description of the Related Art
There is known a fuel vapor emission control system for an engine having a fuel tank, the system having an adsorbing layer of activated carbon for temporarily adsorbing fuel vapor introduced from the fuel tank thereto, wherein air is introduced into the layer so that the air passing therethrough desorbs fuel vapor adsorbed within the layer, and that fuel vapor, together with the air, is purged into the engine.
During, for example, an engine fuelling operation, an amount of fuel vapor generated within the fuel tank is large. Therefore, just after the fuelling operation, an amount of fuel vapor adsorbed in the layer is large. In this situation, however, if the engine operates, and the air is introduced into the layer, a large amount of fuel vapor is desorbed from the primary layer. Accordingly, a large amount of fuel vapor is purged into the engine. This makes it difficult to control the air-fuel ratio in the engine, and the amount of unburned hydrocarbon discharged into the exhaust passage of the engine can become large.
To solve this problem, Japanese Unexamined Utility Model Publication No. 63-198462 discloses a fuel vapor emission control system for an engine having a fuel tank, the system having primary and secondary adsorbing layers for temporarily adsorbing fuel vapor introduced thereto, the primary and the secondary layers being connected in series, wherein the air passed through the primary layer is introduced to the secondary layer so that the air passing therethrough desorbs fuel vapor adsorbed within the primary and the secondary layers therefrom, and that fuel vapor, together with the air, is fed into the engine, and wherein fuel vapor is introduced from the fuel tank to the primary layer. In this system, fuel vapor desorbed from the primary layer is temporarily adsorbed within the secondary layer, and a large amount of fuel vapor is prevented from being purged into the engine.
In this connection, just before the engine fuelling operation, since the amount of liquid fuel in the fuel tank is generally small, liquid fuel in the fuel tank may easily evaporate. In the system described above, fuel vapor generated when the engine is running is also introduced into the primary layer, and fuel vapor desorbed from the primary layer is then introduced to the secondary layer and temporarily adsorbed therein. To this end, when the fuelling operation is to be carried out, the large amount of fuel vapor is adsorbed in the secondary layer. Thus, a problem arises that the adsorbing capacity of the secondary layer can be small when the fuelling operation is to be carried out.